ML/DeepLearning Note2-MLP(多层感知机)

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tips：一般在神经网络中，一层神经网络指的是权重，偏差，以及激活函数；输入层(input layer)一般不纳入其中，因为它只有一个输入

多层感知机的实现

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import torch
from torch import nn, softmax
from d2l import torch as d2l


batch_size = 256
train_iter, test_iter = d2l.load_data_fashion_mnist(batch_size)

num_inputs , num_outputs, num_hiddens = 784,10,256
#第一层是输入层
W1 = nn.Parameter(torch.randn(num_inputs,num_hiddens,requires_grad=True))
#前面两个参数分别对应行数和列数
b1 = nn.Parameter(torch.zeros(num_hiddens,requires_grad=True))
#第二层是输出层
W2 = nn.Parameter(torch.randn(num_hiddens,num_outputs,requires_grad=True))
b2 = nn.Parameter(torch.zeros(num_outputs,requires_grad=True))

params = [W1,b1,W2,b2]

def relu(X):
    a = torch.zeros_like(X)
    #生成一个全0的tensor，和X的形状一致
    return torch.max(X,a)
#模型实现
def net(X):
    X = X.reshape((-1,num_inputs))
    H = relu(torch.matmul(X,W1) + b1)
    return torch.matmul(H,W2) + b2

loss = nn.CrossEntropyLoss()

#模型的训练
num_epochs , lr = 10, 0.1
updater = torch.optim.SGD(params,lr=lr)
d2l.train_ch3(net,train_iter,test_iter,loss,num_epochs,updater)